Over the past few years genome-wide association studies (GWASs) have identified numerous genes associated with common human diseases. In these studies, genetic variation in thousands of individuals is collected and correlated with the disease status in these individuals. A challenging aspect of GWAS is that the collected individuals are related to each other by differing degrees. This can lead to spurious associations which are genes that appear to be associated with the disease, but in fact are an artifact of the relatedness between individuals. Several methods have been proposed to address this problem and are implemented in publicly available software packages. Environmental factors often interact with genetic variation to increase risk of disease. Identifying these interactions, referrd to as gene-by-environment (GxE) interactions, is now a major focus of research in both human studies and model organism studies. Discovering GxE interactions can provide insight into disease pathways, an understanding of the effect of environmental factors in disease, better risk prediction and personalized therapies. Model organisms such as mouse are ideal environments for studying GxE interactions because environmental exposures can be carefully controlled. Unfortunately, for the same reasons that relatedness can cause spurious associations in association studies, relatedness can cause spurious gene-by- environment interactions. In this proposal we propose to develop methodology that corrects for relatedness in studies that search for gene-by-environment interactions. The results of our project will be a set of methods that are can detect gene-by-environment interactions consistently even when the individuals in the study are related. These methods can then be widely used by many researchers involved in studies to discovery gene-by-environment interactions. We will apply our developed methods to the Minnesota Center for Twin and Family Research (MCTFR) data to investigate how gene-environment interplay influences the development of substance abuse (SA) and to mouse genetic studies investigating the genetic factors which influence response to high fat diet and susceptibility to heart failure. We will make implement our methods available to the research community through publicly available software packages and webserver resources.